Now, Harry Potter fans think that if Mark Evans has the same name as Harry's mother, they are probably related. Different fans believe in different theories, here are a few:

Harry and Mark are brothers. I personally think this is impossible as Mark Evans is 10 years old, and as Lily died 14 years before this was said, Mark would have been born 4 years after his mother has died, which is impossible.

Harry and Mark are cousins. There are two options to interpet this:

Mark Evans is Petunia Dursley's son. I think this is wrong because if this was the case:

Harry and Dudley would have seen her pregnant, as they were 5 years old then.

Mark's last name would have been Dursley, not Evans.

Mark Evans is the son of an uncle of Harry's which we do not know of yet. I think this is not correct because we would have met this uncle and Mark already. This is unless Petunia is in bad relationship with him (possibly because he liked Lily). Here's a great option for speculations!

Mark is something further away from Harry, like a second cousin, which is what I believe.